In the prior art, the following two approaches have been proposed for cognitive radio networks. The first approach is the so-called common model. Under this framework, SUs sense the channel in search of temporal and spatial spectrum holes and exploit such detected opportunities to complete their transmission. Another approach, spectrum leasing requires the PU's participation. In this model, the PUs negotiate with the SUs to lease part of their owned spectral resource in exchange for appropriate remuneration.
Recently, a new paradigm termed Cooperative Cognitive Radio Networks (CCRN) is advocated. In CCRN, SUs cooperatively relay the data for the PUs. By exploiting the cooperative diversity, the transmission rate of PUs can be largely improved. In return, The PU leases its channel to SUs for a fraction of time to transmit SUs' data, resulting in a “win-win” situation.
Existing CCRN-based schemes all operate in a time-division fashion. Data is transmitted frame by frame for a primary link. A frame duration is divided into three time slots (phases). The first time slot (phase) is used for the primary transmitter to send data to the relaying SUs. In the second time slot (phase), those SUs form a distributed antenna array to relay the primary data to the primary receiver, improving the throughput of primary link. In return, the third time slot (phase) is then leased to the SUs for their own transmissions.
Although such framework of CCRN improves both the PU and SU's throughput, there still exist some inefficiencies, specifically in two aspects. First, the PU must completely give out its spectrum access to the SUs to transmit the secondary data in the third time slot, as a reward for SUs helping to relay the primary data. The throughput that the SUs can obtain in this phase should compensate for the power that they consumed in the previous relaying communications, otherwise the SUs would not have incentive to participate. Thus, the length of the third time slot should be set reasonably large. This introduces a high overhead to the PUs' communications. Second, from the SUs' point of view, the transmission of their traffic is confined within the third time slot, which limits the throughput they can achieve. Especially considering there may be multiple secondary links competing for spectrum access in the third phase (time slot), the rate for each link is therefore low.